Tests which may be performed for COVID-19 can be divided as follows:
- Tests which specifically look for the virus.
- Tests on the blood that look for evidence of antibodies (IGG) which suggest previous infection or vaccination.
Tests that look for the virus
There are several tests available which check for the COVID-19 virus:
- PCR tests: these are the gold standard tests. They have the highest sensitivity and specificity. These are the tests used in government testing centres.
- Rapid tests (antigen tests or lateral flow tests): these tests are quicker and give results within 15 minutes but they have lower sensitivity and specificity. Rapid tests have been used widely in situations in which Covid infections are more widespread. See here for more information on lateral flow tests.
This is the method of testing used by the Hong Kong government. PCR tests can be performed on a variety of tissue samples. Most commonly we test deep nasal (nasopharyngeal/NP swabs) but deep throat saliva and even stool samples in children can be used.
One disadvantage of PCR tests is that they can remain positive for some time after a person is no longer infectious. There is an argument to say that they are very accurate in telling whether someone has been infected but less accurate at telling whether they are infectious.
There is a legal obligation to report positive PCR tests to the Health Department in Hong Kong. This has implications for individuals (hospital) and close contacts (quarantine).
Lateral Flow Tests/Rapid Tests:
These tests are similar to pregnancy tests in that they give results very quickly. They are less accurate due to lower sensitivity and specificity. This is especially important when a condition is less frequent in a population (see below). These tests have been shown to be more accurate with higher viral load and many public health doctors argue that they are a better measure of infectiousness. There is no legal obligation currently to report a positive lateral flow test in Hong Kong.
When the CHP announce provisional positive results they are NOT using rapid tests. The provisional results are PCR tests from designated and approved private laboratories in Hong Kong. All such tests are then confirmed in a government laboratory.
What are Sensitivity and Specificity?
The sensitivity of a test is a measure of how accurate the test is in measuring a true positive result. The higher this number the more likely that all the true positives have been identified. Specificity is a measure of how accurate the test is in measuring a true negative result. The higher this number the more likely that all the true negatives have been identified. Of course, things are never quite as simple in statistics and as the prevalence (number of cases in the population) falls. The predictive value of these tests also changes. Imagine a test with a 95% specificity looking for a condition that affected 5% of people. Although 95% seems like a high number in this case a false positive case (5%) is the same incidence as a true positive case (5% of the population) which means that any positive test only has a 50% chance of being a true positive and a 50% chance of a being the wrong result.
There are also blood tests which measure antibodies against COVID-19 IGG. An IGG test becomes positive within the first few weeks of the illness or vaccination. We now have significant experience with antibody levels. We know for instance that antibody levels are significantly higher after BioNTech vaccination than Sinovac and we know that they wane, typically over six months and are significantly improved by booster doses. We have not yet identified a threshold at which illness does not occur but we have very good evidence internationally that serious illness is significantly lower in people who have positive antibodies.
The Hong Kong Health department has set up testing centres in which anybody without symptoms can be tested for COVID-19. There is no charge for testing in the government centres except when taken for travel purposes. Further information about the government testing program can be found online.
Test, trace and isolate in addition to social distancing measures has been and will remain the mainstay of management of COVID-19 until high vaccination levels are achieved, especially in the most vulnerable members of the population. If the epidemic increases in size and Hong Kong moves away from a Zero Covid policy we can expect an increasing use of rapid tests in addition to PCR testing in order to mitigate the number of cases in the community.